The production cost of micro LED displays is very sensitive to the transfer tact time of micro LEDs from the substrate on which the micro LEDs are formed (i.e., a donor substrate) to the thin-film-transistor (TFT) backplanes on the substrate on which the micro LED display is formed (i.e., the host substrate), especially for large size displays (such as tablets or notebooks). Each pixel in an LED display typically includes at least a red LED, a green LED, and a blue LED. The different colored LEDs each require different processing conditions and materials. In the case of active matrix micro LED display panels, GaN-based micro LEDs that emit red, green, and blue colors are fabricated on different donor substrates. The different colored LEDs then need to be transferred from their donor substrates to thin-film-transistor (TFT) backplanes on the host substrate to make red, green, blue (RGB) pixels. Since the different LED colors are formed on different host substrates, the transfer process has to be executed three times to accommodate each of the three colors, thereby further increasing the overall transfer time and increasing production cost.
Furthermore, the acceptable defect density of displays needs to be approximately 1 pixel per million (ppm). In order to achieve this defect density requirement, two micro LEDs for each color (red, green, and blue) may be used in each pixel to provide redundancy. However, transferring more micro LEDs results in higher manufacturing costs due to the increased transfer time.
One approach to meet both cost and defect density requirements is to manufacture monolithic red, green, and blue LED pixels on the host wafers then transfer the whole pixel (as opposed to transferring individual micro LEDs with different colors). However, manufacturing three colors monolithically on the same wafer is challenging, especially when the maximum power efficacy of individual red, green, and blue LEDs require different processing temperatures and other metalorganic vapor phase epitaxy (MOVPE) process parameters.